Resistor banks

ABSTRACT

A Resistor bank has first and second supporting member. Held on to the supporting members is a resistor element comprising a strip of resistor material which is bent into a zigzag shape. The strip is continuous and has a non-planar cross section profile. A plurality of insulating spacers mounted end to end on each of the first and second supporting members isolate the element from the first and second supporting members. The engaging surfaces of adjacent spacers are of a complimentary shape thereby to enable each spacer to move relative to each adjacent spacer with any flexing of the relevant one of the supporting members. The element is held by a plurality of clips each at a bend thereof on to a respective one of the supporting members. Each clip has at least one fastening lug for insertion into at least one fastening hole in a corresponding bend in the element.

This application is a national stage application filed under 35 U.S.C.371, claiming priority to International Application No. PCT/GB00/03824,filed on Oct. 6, 2000, which claims priority to UK Patent ApplicationNo. 9923847.9 filed on Oct. 9, 1999.

FIELD OF THE INVENTION

The invention relates to resistor banks of the type used in dynamicbraking resistor systems for traction vehicles.

BACKGROUND OF THE INVENTION

Electric motor driven traction vehicles, such as trains, trams etc., areknown to have two braking devices; a friction braking device comprising,for example, friction blocks or pads applied to the wheels or axlesunder the control of compressed air or hydraulics, and a dynamic brakingdevice comprising a resistor system which is applied as an electricalload on the traction motor, thereby retarding its rotation.

Dynamic braking resistor systems are known to comprise a number ofresistor banks each having a pair of supporting members with a resistorelement held therebetween. The element may be constructed from severalparts which may be joined together by welding and bent into a zigzagshape. The bent element is held on to the supporting members by clips ateach of the bends and flanges at the joins, through which the supportingmembers extend. An electrical connection is made to each end of theelement and insulating spacers isolate the element from the supportingmembers.

The need to weld parts together increases manufacturing time, andtherefore cost, and welds may be liable to failure and may compromisereliability. In addition, there are now available resistor alloys whichare cheaper than those previously available, and which have goodtemperature stability characteristics. However, some of these new alloysare not well adapted to welding.

The clips which are known to be used to hold resistor elements to thesupporting members may have lugs which are crimped around the edges ofthe element. However, these have been found inadequately to hold theelement which may, at the very least, slide in the clip, rub against theelement edge and work loose.

Resistor banks in traction vehicles may be subject to intense mechanicalstresses and vibrations which may, at the very least, cause flexing ofthe supporting members. Generally, insulating spacers for isolating theelement from the supporting members are mounted end to end on each ofthe supporting members. The engaging faces of adjacent spacers areusually plane. In the event of large-scale flexure of the supportingmembers, adjacent spacers may effectively part company so that crevicesare created therebetween. These crevices are vulnerable to the ingressof containments whose presence may compromise the electricallyinsulating property of the spacers.

EP-A-0676774 discloses a resistor bank having a first and secondsupporting members, a resistor element comprising a strip of resistormaterial which is bent into a zigzag shape and holding means for holdingthe element at a bend thereof on to a respective one of the supportingmembers. EP-A-0676774 also discloses insulating spacers for isolatingthe element from the supporting members.

SUMMARY OF THE INVENTION

The invention provides a resistor bank having first and secondsupporting members, a resistor element comprising a strip of resistormaterial which is bent into a zigzag shape, and holding means comprisinga plurality of clips each for holding the element at a bend thereof onto a respective one of the supporting members, characterised in thateach clip has at least one fastening lug for insertion into at least onefastening hole in a corresponding bend in the element.

Having clips with at least one lug which is received in a hole in a bendin the element provides much better integrity, in the sense of making amore rigid connection, between the element and a respective one of thesupporting members in comparison to clips with lugs which crimp over theedges of the element. Also clips with lugs for insertion into holesprovide much better lateral support than clips with lugs which crimpover the edges of the element, and there is no rubbing between the clipand the element edge, which can cause the clip to wear.

Each clip may extend around the narrower diameter section of one of thespacers. With the spacers arranged in an end to end fashion so that thewider diameter section of one spacer abuts the narrower diameter sectionof the adjacent spacer, the clip is restrained from lateral movement bythe two neighbouring wider diameter sections.

The surface of the or each clip adjacent the or each lug which engages acorresponding bend in the element has substantially the same curvatureas the corresponding bend. Having complimentarily curved bend engagingsurfaces contributes to the integrity and the lateral support offered bythe clip.

The clip may be of stainless steel.

Preferably, each clip has two fastening lugs and there are two fasteningholes in the corresponding bend, one fastening hole for each fasteninglug.

An advantage of having a continuous element, without any joins, is thatthe number of welds required in the construction of the bank isminimised (preferably only two to the element: one to each of twoconductors, one at each end of the element), and production time andcosts consequently benefit. In addition, a minimisation of weldingfacilitates the adoption of the more recently available, cheaperresistor alloys which do not weld easily or require specialist weldingtechniques. The element may be of NiCr or FeAlCr or any other suitableresistor alloy. The joins and any flanges in multi-part elements alsoprovide a focus for arcing in use, so not having joins reduces thesusceptibility to arcing.

A drawback of having a continuous element is the tendency of some alloyresistor materials to distort when exposed to high temperatures. Thistendency to distort is commonly known as creepage. In a traction vehicledynamic braking resistor system, temperatures may reach 700° C., whichnecessitates that the element should be as rigid as possible in order toresist the affects of creepage whilst allowing the gap between adjacentlegs of the element to be minimised. Space is an important considerationfor dynamic braking resistor systems which may comprise fifty resistorbanks, and minimising the gap between adjacent legs of the element is akey factor in space considerations. Also to be borne in mind is that theeffect of adjacent legs of the element contacting and causing a shortcircuit can be very damaging. However, rigidity of the element may beimproved over and above that offered by a continuous element of planarcross sectional profile by having a non-planar cross sectional profile.The element is usually air cooled, and a non-planar cross sectionalprofile must not compromise air flow between the legs of the element.Preferably, the cross sectional profile of the element is of a shapewhereby at least one region of the element protrudes further away from adatum than at least one other region of the element. Further preferably,one region protrudes in one direction away from the datum and tworegions of the element protrude in an opposite direction away from thedatum. Alternatively, a plurality of regions may protrude in onedirection away from the datum and a plurality of regions may protrude inan opposite direction away from the datum.

The invention further provides a resistor bank having first and secondsupporting members, a resistor element comprising a strip or resistormaterial which is bent into a zigzag shape, insulator means comprising aplurality of insulating spacers for isolating the element from thesupporting members mounted end to end on each of the first and secondsupporting members, characterised in that the engaging surfaces ofadjacent spacers are of complimentary shapes thereby to enable eachspacer to move relative to each adjacent spacer with any flexing of therelevant one of the first and second supporting members. Relativemovement between adjacent spacers minimises the occurrence of crevicesbetween the spacers. The relative movement also assists in reducing thefriction between adjacent spacers. Such friction results in energylosses in the bank which are detrimental to its performance.

Preferably, the engaging surfaces of adjacent spacers are ofcomplimentary curved shapes. Further preferably, the engaging surfacesare spherically curved. The radius of curvature of the engaging surfacesare selected so as to be substantially the same, although the radius maybe so large that the surfaces are not discernibly curved.

The first and second supporting members may each comprise a cylindricalrod and each spacer may have a rod receiving bore so as to be slidablyreceivable on a respective one of the rods. Spring means may be providedto exert such a force as to urge the spacers together. Accordingly, thespacers are prevented from rattling together in use when the bank issubjected to vibrational stresses. Preferably, the force exerted by thespring means is maintained substantially constant despite the thermalexpansion which occurs in use to the supporting members and/or spacers.Further preferably, the spring means comprises a coil spring, coiledaround one of the supporting members.

Each spacer may comprise two substantially cylindrical sections each ofdifferent diameter and the spacers may be mounted whereby the narrowerdiameter section of each spacer abuts the wider diameter section of eachadjacent spacer.

The insulator means may further comprise a layer of insulating materialon each of the first and second supporting members, between the firstand second supporting members and the spacers.

The spacers may be of ceramics materials, such as corderite ceramic,which provides electrical and heat insulation.

The invention also provides a resistor bank having first and secondsupporting members, a resistor element comprising a strip of resistormaterial which is bent into a zigzag shape, holding means comprising aplurality of clips each for holding the element at a bend thereof on toa respective one of the members, and insulating spacers for isolatingthe element from the supporting members mounted end to end on each ofthe first and second supporting members, characterised in that each cliphas at least one fastening lug for insertion into at least one fasteninghole in a corresponding bend in the element, and the engaging surface ofadjacent spacers are of complimentary shapes thereby to enable eachspacer to move relative to each adjacent spacer with any flexing of therelevant one of the first and second supporting members.

The invention will now be described, by way of example, with referenceto the following drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a resistor bank according to the invention foruse in a traction vehicle dynamic braking resistor system;

FIGS. 2A and 2B are side and narrower diameter end views respectively ofan insulating spacer used in the resistor bank shown in FIG. 1;

FIGS. 3A and 3B are side and cross sectional views (taken along lineIII—III) respectively of a holding clip used in the resistor bank shownin FIG. 1;

FIGS. 4A and 4B are alternative cross sectional profiles for a resistorelement used in the resistor bank shown in FIG. 1;

FIG. 5 is a side view of the end of one of the rods of the resistor bankshown in FIG. 1;

FIG. 6 is a perspective view of a holding clip used in the resistorbank:

FIG. 7 is a perspective view of a holding clip fastened to the hairpinbend of the resistor element;

FIG. 8 is a perspective view of a holding clip fastened to the hairpinbend of the resistor element,

FIG. 9 is a perspective view of a holding clip fastened to the hairpinbend of the resistor element;

FIG. 10 is a side view of a holding clip fastened to the hairpin bend ofthe resistor element and wrapped around a spacer; and

FIG. 11 is a top view of a holding clip fastened to the hairpin bend ofthe resistor element and wrapped around a spacer.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a resistor bank for a traction vehicle dynamicbraking resistor system is indicated generally at 1. The bank 1comprises first and second supporting members 2, 4 in the form ofcylindrical steel rods commonly known as tie rods. Over each of the rods2, 4 is a thin tube of silicone bonded micanite (not shown) whichprovides insulation. Mounted on each of the rods 2, 4, over the micanitetube, are a plurality of ceramics material, insulating spacers 6 whichare each “top hat” shaped, that is to say, each spacer 6 comprises twocylindrical sections 20, 22, each of different diameter (see FIGS. 2Aand 2B). Each spacer 6 has a central, rod receiving bore 24 of adiameter slightly greater than the diameter of the rods 2, 4, and thespacers 6 are slidably received on the rods 2, 4. The spacers 6 aremounted end to end so that the wider cylindrical section 22 of onespacer 6 abuts the narrower cylindrical section 20 of the adjacentspacer 6. Nuts 10 are threaded on to threaded lengths at the ends ofeach rod 2, 4 to hold the insulators captive therebetween. Between thenuts 10 at one end of the rods 2, 4 respectively and the adjacent,outermost spacers 6, springs 44 are coiled around the rods 2, 4respectively so as to exert an axial compression on the spacers 6 andurge them tightly together. One of the springs 44 is shown in greaterdetail in FIG. 5. Clearly, visible in FIG. 5 are spacer washers 46between the spring ends and the nut 10 and the outermost spacer 6respectively.

The rods 2, 4 are held spaced apart in parallel in a frame (not shown)which also holds other resistor banks (not shown) which together make upa resistor system. Extending between each end of the rod 2 and thecorresponding end of the other rod 4 are conductors 12, 14. At each endof each conductor 12, 14 there is an aperture through which the relevantrod 2, 4 extends, and each conductor 12, 14 is held in place between twoof the spacers 6. The conductors 12, 14 are selectively connected, thatis, when braking is required, to the respective terminals of a tractionvehicle motor (not shown).

Between the conductors 12, 14 is a resistor element 16 in the form of acontinuous, that is, without join, strip of resistor alloy, such as30/20 NiCr, which is bent into a zigzag shape so as to follow aserpentine course back and forth between the rods 2, 4. The element 16is MIG welded at each end to a respective one of the conductors 12, 14.A plurality of clips 26 (see FIGS. 3A and 3B), one at each hairpin bend18, hold the element 16 to a respective one of the rods 2, 4.

With reference also to FIG. 2, each spacer 6 is made from heat andelectrically insulating corderite ceramic. The annular, end surface 28of the narrower cylindrical section 20 of each spacer 6 is sphericallycurved. The end surface 30 around the bore 24 of wider cylindricalsection 22 of the spacer 6 is also curved in a complimentary sphericalfashion, with substantially the same radius of curvature. Thus, whenmounted on the rods 2, 4 in an end to end manner, the complimentarilyshaped surfaces 28, 30 of adjacent spacers 6 engage and mate together.The curvature of the mating surfaces 28, 30 enables adjacent spacers 6to move relative to one another, in what is effectively a slidingmotion, with the transverse flexing of the rods 2, 4 which occurs in useas a result of mechanical stresses. This movement occurs withoutexposing any crevices between adjacent spacers 6.

With reference also to FIG. 3, each clip 26 is generally U-shaped having(in the unused state) two lugs 32 which are a partial continuation ofthe legs 34 of the U. The remainder of the material which forms thecontinuation of the legs 34 is bent substantially perpendicularly to thelugs 32 to form element engaging surfaces 36. In use, the U of each clipfits around the narrower section 20 of a relevant spacer 6 and isrestrained from lateral movement by the neighbouring wider sections 22.Each lug 32 is received in one of two holes in the corresponding hairpinbend 18 of the element 16. Thus, the element engaging surfaces 36 arebrought in to contact with the corresponding hairpin bend 18. Theelement engaging surfaces 36 are saddle shaped, having substantially thesame curvature as the hairpin bend 18 so that, with the lugs 32 bentback towards the element engaging surfaces 36 in the operating position,the clip 26 is fast and rigidly fixed to the element 16.

With reference also to FIG. 4, the element 16 does not have a planarcross sectional profile, but is shaped so as to improve upon the lack ofrigidity of a plane profile. The element 16 is pressed such that, takingits original axis of symmetry as a datum D, a central region 38 of theelement 16 protrudes in one direction away from the datum D and theperipheral regions 40, 42 to either side of the central region 38protrude in an opposite direction away from the datum D. The transitionfrom the maximum protrusion of one region to the maximum protrusion inthe next region may be gradual, as in the example shown in FIG. 4 a, orsudden, as in the example shown in FIG. 4 b. The profile promotesrigidity in the element 16 without compromising cooling airflow betweenthe adjacent legs of its serpentine course.

1. A resistor bank, comprising a resistor element formed by a strip ofresistor material which is bent into a zigzag shape so as to have aplurality of bent regions at opposing sides, these bent regions eachhaving at least one fastening hole formed therein, first and secondsupporting members disposed adjacent the respective opposing sides ofthe resistor element, insulating means mounted on each of the supportingmembers for isolating the element from the supporting members andholding means for holding the element at each bent region thereof on toa respective one of the supporting members, the improvement wherein theholding means comprises a plurality of generally U-shaped clips whichfit around the insulating means on the supporting means, each cliphaving at least one fastening lug for insertion into the at least onefastening hole in a corresponding bent region of the element, whereineach clip also has a surface adjacent to each lug which is bent tosubstantially the same curvature as the corresponding bent region andwherein the element is securely retained by engagement of the respectivelugs through the respective fastening holes followed by bending back ofthe lugs towards the adjacent curved surfaces.
 2. A resistor bankaccording to claim 1 wherein the clip is of stainless steel.
 3. Aresistor bank according to claim 1 wherein each clip has two fasteninglugs and there are two fastening holes in a corresponding bend in theelement, one fastening hole for each fastening lug.
 4. A resistor bankaccording to claim 1 having insulator means comprising a plurality ofinsulating spacers mounted end-to-end on each of the support members. 5.A resistor bank according to claim 4 wherein the engaging surfaces ofadjacent spacers are of a complimentary shape thereby to enable eachspacer to move relative to each adjacent spacer with any flexing of arelevant one of the supporting members.
 6. A resistor bank according toclaim 5 wherein the engaging surfaces of adjacent spacers are ofcomplimentary curved shapes.
 7. A resistor bank according to claim 6wherein the engaging surfaces of adjacent spacers are sphericallycurved.
 8. A resistor bank according to claim 7 wherein spring means isprovided for urging the spacers on each supporting member together.
 9. Aresistor bank according to claim 8 wherein the first and secondsupporting members each comprise a cylindrical rod, and each spacer hasa rod receiving bore.
 10. A resistor bank according to claim 9 whereineach spacer comprises two substantially cylindrical sections each ofdifferent diameter and the spacers are mounted whereby the narrowerdiameter section of each spacer abuts the wider diameter section of eachadjacent spacer.
 11. A resistor bank according to claim 10 wherein eachclip extends around the narrower diameter section of one of the spacers.12. A resistor bank according to claim 11 wherein the insulator meansfurther comprises a layer of insulating material on each of the firstand second supporting members.
 13. A resistor bank according to claim 12wherein the spacers are of corderite ceramic or other ceramic materials.14. A resistor bank according to claim 13 wherein the element iscontinuous and has a non-planar cross-sectional profile.
 15. A resistorbank according to claim 14 wherein the cross-sectional profile of theelement is of a shape whereby at least one region of the elementprotrudes further away from a datum than at least one other region ofthe element.
 16. A resistor bank according to claim 15 wherein oneregion of the element protrudes in one direction away from the datum andtwo other regions of the element protrude in an opposite direction awayfrom the datum.
 17. A resistor bank according to claim 16 wherein theelement is of NiCr or FeAlCr or another resistor alloy.